The good thing about web-site cookies is that they are blockable. Cookies are the devil-you-know and web browsers are set up to deal with/delete them.

Now there’s a new insidious devil in town called canvas fingerprinting.

A new, extremely persistent type of online tracking is shadowing visitors to thousands of top websites, from WhiteHouse.gov to YouPorn.com.

First documented in a forthcoming paper by researchers at Princeton University and KU Leuven University in Belgium, this type of tracking, called canvas fingerprinting, works by instructing the visitor’s Web browser to draw a hidden image. Because each computer draws the image slightly differently, the images can be used to assign each user’s device a number that uniquely identifies it.

Like other tracking tools, canvas fingerprints are used to build profiles of users based on the websites they visit — profiles that shape which ads, news articles, or other types of content are displayed to them.

But fingerprints are unusually hard to block: They can’t be prevented by using standard Web browser privacy settings or using anti-tracking tools such as AdBlock Plus.

In a nutshell, canvas fingerprinting detects subtle variations in the way your specific browser renders a requested image to create a fingerprint for your specific computer.

The researchers found canvas fingerprinting computer code, primarily written by a company called AddThis, on 5 percent of the top 100,000 websites.

Here’s a link to the AddThis site, and another to their opt out page (which uses cookies as an opt out mechanism!).